Communication System

1. A communication system wherein a slave device and a master device configured to transmit and receive data to/from the slave device by specifying an address are connected by a clock signal line configured to convey a clock transmitted from the master device and a serial data signal line configured to bidirectionally transmit data, the communication system comprising: a first signal line configuring a portion of the serial data signal line and configured to transmit predetermined data from the master device to the slave device; a second signal line configuring a portion of the serial data signal line and configured to transmit predetermined data from the slave device to the master device; a signal line selection portion configured to select either one of the first signal line and the second signal line so as to be effective as the serial data signal line; and a signal control portion including a terminal configured to output a signal direction switching signal for controlling the selecting operation in the signal line selection portion, wherein the signal line selection portion is configured by a first signal line selection portion and a second signal line selection portion, the first signal line selection portion includes a first OR circuit and a first three-state buffer circuit provided on the first signal line, and a first buffer provided on the second signal line, and the second signal line selection portion includes a second OR circuit and a second three-state buffer circuit provided on the second signal line, a second buffer provided on the first signal line, and an inverter circuit provided between the terminal of the signal control portion and the second OR circuit.

2. The communication system according to claim 1 , wherein the signal control portion controls the selecting operation in the signal line selection portion to select either one of the first signal line and the second signal line so as to be effective as the serial data signal line, according to a kind of the data transmitted and received between the master device and the slave device in the serial data signal line.

3. The communication system according to claim 1 , wherein the signal control portion controls the selecting operation in the signal line selection portion to select the first signal line so as to be effective as the serial data signal line when the predetermined data is transmitted from the master device to the slave device, and to select the second signal line so as to be effective as the serial data signal line only at a time of a reception bit at which the master device receives the predetermined data from the slave device.

4. The communication system according to claim 1 , wherein the signal control portion outputs the signal direction switching signal at a predetermined signal level to the signal line selection portion in a case that the first signal line is effective as the serial data signal line, and outputs the signal direction switching signal for which the predetermined signal level is inverted to the signal line selection portion in a case that the second signal line is effective as the serial data signal line.

5. The communication system according to claim 1 , wherein an output terminal of the second buffer is connected to one of two input terminals of the first OR circuit, and the terminal of the signal control portion is connected to another of the two input terminals of the first OR circuit.

6. The communication system according to claim 1 , wherein an output terminal of the first OR circuit is connected to an input terminal of the first three-state buffer circuit, and the terminal of the signal control portion is connected to a control input terminal of the first three-state buffer circuit.

7. The communication system according to claim 6 , wherein the first three-state buffer circuit controls an output signal from the output terminal, according to the signal direction switching signal outputted from the terminal of the signal control portion.

8. The communication system according to claim 1 , wherein the terminal of the signal control portion is connected to an input terminal of the inverter circuit, an output terminal of the first buffer is connected to one of two input terminals of the second OR circuit, and an output terminal of the inverter circuit is connected to another of the two input terminals of the second OR circuit.

9. The communication system according to claim 1 , wherein an output terminal of the second OR circuit is connected to an input terminal of the second three-state buffer circuit, and the output terminal of the inverter circuit is connected to a control input terminal of the second three-state buffer circuit.

10. The communication system according to claim 9 , wherein the second three-state buffer circuit controls an output signal from the output terminal, according to the signal direction switching signal outputted from the terminal of the signal control portion and inverted by the inverter circuit.